To avoid DNS censorship, my ISP filters pirate bay, plenty of other sites.
Because I have frequently seen provably-wrong results from other resolvers, and some highly-suspicious results from 8.8.8.8 on occasion (though I haven't checked particularly often).
The point being, unless you're proving the results with DNSSEC (or similar), you can't trust any source.
I use and recommend doing both, as there's very little downside to running your own resolver.